Genome size (amount of chromosomal protein plus DNA, per cell) and radiosensitivity for mutation induction and for cell killing will be measured in different species and in different tumor cell culture lines, and correlations tested. Genome size is a good predictor of relative radiosensitivity. Several human and animal tumor cell lines of different radiosensitivities will be examined for their genome size and cytological characteristics, to see if these differences account for their different sensitivities. The same tumor lines will also be grown in tissue culture as "solid" microsphere colonies, to test the effect of solid vs. single-cell growth on radiosensitivity. Microsphere colonies simulate in vivo solid tumors.